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The name "microbe" suggests creatures that are so tiny you need a microscope to see them. For most microbes this is true, but there are some gargantuan microbes that we can actually see with the unaided eye. On this page, you’ll read about the biggest of the big in each of the main categories of microbes: viruses, bacteria, protists, fungi and, as an extra added bonus, parasitic worms.

Largest Virus

Viruses are super tiny. There’s not a single type of virus that can be seen without a microscope and regular light microscopes that allow us to see bacteria and protists aren’t powerful enough to view viruses. Scientists use special types of electron microscopes to see viruses.

However, viruses come in a wide range of sizes and there are some biggies among them (big being a very relative term here). The largest of them all are a group called the Filoviridae <fill-oh-vir-ih-dee>. This family includes the Ebola virus, which you might have heard of. Filoviridae are long, thin viruses that look like bent tubes. They can reach up to 1,000 nanometers or 1/10,000th of a centimeter in length. This makes them actually longer than some of the smaller bacteria. However, these long viruses are only about 80 nanometers in diameter.

Filoviridae are big for viruses, but they are mere pipsqueaks next to the largest of the bacteria.

Biggest Bacterium

The colossus among bacteria is a single-celled giant that lives in the ocean and is named Thiomargarita namibiensis <thigh-oh-mar-ger-ee-tuh nah-mih-bee-en-sis>, which means "sulfur pearl of Namibia." It was found in the ocean floor off the coast of Namibia in Africa. T. namibiensis’s ball-shaped cells can grow to almost 1 millimeter or 1/25th of an inch in diameter. That’s about as big as the period at the end of this sentence. Size-wise, the largest T. namibiensis cell is to an ordinary bacterium what a 75-foot (23-meter) blue whale is to a newborn mouse. Wow!

T. namibiensis "eats" sulfur and "breathes" nitrate. It stores these molecules in bubble-like compartments in its cell called vacuoles <vac-you-ohls>. These vacuoles take up 97 percent of the space inside the cell and give the bacterium a pearly, blue-green color. Basically, these vacuoles act like food storage and scuba gear, enabling the bacterium to survive long periods when sulfur and nitrate are scarce without "starving" or "suffocating."

A bacterium that’s big enough to be seen without the aid of a microscope is pretty amazing. But it’s a mere dust speck when it comes to the largest protozoan…

Giants Among Protozoa

The biggest single-celled protozoa are ocean dwelling creatures called foraminifera <for-am-an-if-air-uh> or forams for short. While many species of foraminifera don’t grow over a single millimeter in diameter, the largest forams can reach sizes of 5 to 6 centimeters (a little over 2 inches) in diameter. (Fossils of ancient forams have reached up to 15 centimeters!) Holy microbe! How can a single cell get that HUGE? It’s thanks in part to shells.

Forams build shells called tests that can support the large size that some of these creatures reach. The tests are commonly divided into chambers that are added as the protozoan grows. The foram cell typically occupies all but the one or two most recently added chambers, the last of which opens out to the seawater. In their simplest forms, tests may be little more than hollow balls or open tubes. But some foram tests can take very complex shapes. Depending on the species, foram tests may be made of sand grains, organic compounds, and other particles cemented together, or they may be made of crystalline calcite, the same mineral that sea shells are made of.

The chambers in larger foram tests often serve as a home for algae and cyanobacteria as well. The forams "farm" these microbes for the compounds they produce as they turn sunlight into energy. Forams also eat bacteria, diatoms, other protozoa and even small animals such as copepods. In turn, forams are an important food source for snails, sand dollars, fish and other creatures. Forams move about and catch their food using hairlike extensions of their cell fluid called reticulopodia <reh-tick-you-low-pod-ee-uh> that poke out of their tests (white "threads" in color photo above). These extensions are like the pseudopods <sue-doh-pods>, or oozing "false feet" that amoebas form, only they’re much thinner and more numerous.

Ok, so 2-inch wide protozoa are definitely huge on the microbial scale, but forams don't hold a candle to the giants among fungi…

Humongous Fungus

As you may have read on the fungi page of this site, fungi straddle the realms of microbiology and macrobiology. Some species are single-celled creatures that can’t be seen without a microscope. Yeast is an example. Other species form the multi-celled structures that we know as mushrooms. Obviously, these can be seen perfectly fine without a microscope.

So you might be thinking that there are lots of big fungi. True. But there’s one that’s bigger than all the rest, that forms structures that spread for miles and that kills large trees. It’s called Armillaria ostoyae <are-mill-are-ee-uh ah-stoy-ee>, known more commonly as the honey mushroom. Read about it in the News section of the site.

A fungus that can cover an area larger than 1,600 football fields definitely takes the prize for biggest of the big. But for pure grossness, nothing beats the colossus among parasites…

Gruesome Giant Parasite

Sometimes the human body is invaded by parasitic creatures that steal food from our guts or dine on our blood. Although many of these parasites are multi-celled creatures, microbiologists study them, too, because most species are too small to see easily without a microscope. However, some can get big—VERY big. The king of these parasites is the beef tapeworm, which can easily grow 7½ meters or 25 feet long! Imagine that living in your guts!

Tapeworms have a head called a scolex <skoh-lecks> with suckers or hooks that allow it to attach itself to the walls of its host’s guts. Tapeworms don’t have stomachs or intestines themselves. Instead they steal digested food passing through the host’s digestive tract. They absorb this food through their bodies. The body of a tapeworm is made up of dozens if not hundreds or thousands of segments, each of which can grow into a brand new tapeworm.

You may have heard wild stories that some models or Hollywood starlets have swallowed tapeworms on purpose as a way to stay thin. This is an urban legend, totally untrue. Even if someone wanted to try this, it wouldn’t really work too well. Tapeworms don’t necessarily go for the fats and sugars. They need useful nutrients so they mainly absorb vitamins and other vital nutrients from the food their hosts eat, depriving their hosts’ bodies of these essential things. So all having a tapeworm does is gradually make a person sicker and sicker.

So there you have it: the giants among the microbes. Pretty amazing how big some of these tiny creatures can get. But that just goes to show how diverse, unusual and wonderful life on Earth is.